This invention relates to a process to produce hollow fiber porous membranes from perfluorinated thermoplastic polymers. More specifically, this invention relates to a process to produce microporous membranes having an essentially skin-free surface on at least one of the inner and outer surfaces, and to the membranes produced.
Microporous membranes are used in a wide variety of applications. Used as separating filters, they remove particles and bacteria from diverse solutions such as buffers and therapeutic containing solutions in the pharmaceutical industry, ultrapure aqueous and organic solvent solutions in microelectronics wafer making processes, and for pre-treatment of water purification processes. In addition, they are used in medical diagnostic devices, where their high porosity results in advantageous absorption and wicking properties.
Hollow fiber membranes are also used as membrane contactors, typically for degassing or gas absorption applications. Contactors bring together two phases, i.e., two liquid phases, or a liquid and a gas phase for the purpose of transferring a component from one phase to the other. A common process is gas-liquid mass transfer, such as gas absorption, in which a gas or a component of a gas stream is absorbed in a liquid. Liquid degassing is another example, in which a liquid containing dissolved gas is contacted with an atmosphere, a vacuum or a separate phase to remove the dissolved gas. In an example of conventional gas absorption, gas bubbles are dispersed in an absorbing liquid to increase the gas/liquid surface area and increase the rate of transfer of the species to be absorbed from the gas phase. Conversely, droplets of liquid can be sprayed or the liquid can be transported as a thin film in counter-current operation of spray towers, packed towers, etc. Similarly, droplets of an immiscible liquid can be dispersed in a second liquid to enhance transfer. Packed columns and tray columns have a deficiency as the individual rates of the two phases cannot be independently varied over wide ranges without causing flooding, entrainment, etc. If however, the phases are separated by a membrane, the flow rates of each phase can be varied independently. Furthermore, all the area is available, even at relatively low flow rates. Due to these advantages, hollow fiber membranes are increasingly being used in contactor applications.
Hydrophobic microporous membranes are commonly used for contactor applications with an aqueous solution that does not wet the membrane. The solution flows on one side of the membrane and a gas mixture at a lower pressure than the solution flows on the other. Pressures on each side of the membrane are maintained so that the liquid pressure does not overcome the critical pressure of the membrane, and so that the gas does not bubble into the liquid. Critical pressure, the pressure at which the solution will intrude into the pores, depends directly on the material used to make the membrane, inversely on the pore size of the membrane, and directly on the surface tension of the liquid in contact with the gas phase. Hollow fiber membranes are primarily used because of the ability to obtain a very high packing density with such devices. Packing density relates to the amount of useful filtering surface per volume of the device. Also, they may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application. Typical applications for contacting membrane systems are to remove dissolved gases from liquids, xe2x80x9cdegassingxe2x80x9d; or to add a gaseous substance to a liquid. For example, ozone is added to very pure water to wash semiconductor wafers.
Porous contactor membranes are preferred for many applications because they will have higher mass transfer than nonporous membranes. For applications with liquids having low surface tensions, smaller pore sizes will be able to operate at higher pressures due to their resistance to intrusion. For applications in which the gas to be transferred in highly soluble in the liquid phase, the mass transfer resistance of skinned membranes is a detriment to efficient operation.
Z. Qi and E. L. Cussler (J. Membrane Sci. 23(1985) 333-345) show that membrane resistance controls absorption of gases such as ammonia, SO2 and H2S in sodium hydroxide solutions. This seems generally true for contactors used with strong acids and bases as the absorption liquid. For these applications, a more porous contactor membrane, such as a microporous membrane, would have an advantage, because the membrane resistance would be reduced. This would be practical if the liquid does not intrude the pores and increase resistance. With the very low surface tension materials used in the present invention, this would be possible without coating the surface of the fibers with a low surface tension material, which is an added and complex manufacturing process step.
An advantage for contacting applications is that the very low surface tension of these perfluorinated polymers allows use with low surface tension liquids. For example, highly corrosive developers used in the semiconductor manufacturing industry may contain surface tension reducing additives, such as surfactants. These developers could not be degassed with typical microporous membranes because the liquid would intrude the pores at the pressures used and permeate, causing solution loss and excess evaporation. In addition, liquid filling the pores would greatly add to the mass transfer resistance of gas transport. U.S. Pat. No. 5,749,941 describes how conventional hollow fiber membranes of polypropylene or polyethylene cannot be used in carbon dioxide or hydrogen sulfide absorption into aqueous solutions containing an organic solvent without the use of a solution additive to prevent leakage. While PTFE membranes would work in these applications, presumably because of their lower surface tension, they are difficult to process into hollow fibers. The membranes of the present invention are made from polymers having similar surface tension properties to PTFE and are more readily manufactured into small diameter hollow fiber membranes.
Microporous membranes have a continuous porous structure that extends throughout the membrane. Workers in the field consider the range of pore widths to be from approximately 0.05 micron to approximately 10.0 microns. Such membranes can be in the form of sheets, tubes, or hollow fibers. Hollow fibers have the advantages of being able to be incorporated into separating devices at high packing densities. Packing density relates to the amount of useful filtering surface per volume of the device. Also, they may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application.
A hollow fiber porous membrane is a tubular filament comprising an outer diameter, an inner diameter, with a porous wall thickness between them. The inner diameter defines the hollow portion of the fiber and is used to carry fluid, either the feed stream to be filtered through the porous wall, or the permeate if the filtering is done from the outer surface. The inner hollow portion is sometimes called the lumen.
The outer or inner surface of a hollow fiber microporous membrane can be skinned or unskinned. A skin is a thin dense surface layer integral with the substructure of the membrane. In skinned membranes, the major portion of resistance to flow through the membrane resides in the thin skin. In microporous membranes, the surface skin contains pores leading to the continuous porous structure of the substructure. For skinned microporous membranes, the pores represent a minor fraction of the surface area. An unskinned membrane will be porous over the major portion of the surface. The porosity may be comprised of single pores or areas of porosity. Porosity here refers to surface porosity, which is defined as the ratio of surface area comprised of the pore openings to the total frontal surface area of the membrane. Microporous membranes may be classified as symmetric or asymmetric, referring to the uniformity of the pore size across the thickness of the membrane. In the case of a hollow fiber, this is the porous wall of the fiber. Symmetric membranes have essentially uniform pore size across the membrane cross-section. Asymmetric membranes have a structure in which the pore size is a function of location through the cross-section. Another manner of defining asymmetry is the ratio of pore sizes on one surface to those on the opposite surface.
Manufacturers produce microporous membranes from a variety of materials, the most general class being synthetic polymers. An important class of synthetic polymers are thermoplastic polymers, which can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application to which the membrane is being used become more severe, the materials that can be used becomes limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken most common polymeric membranes. The high temperature stripping baths in the same industry consist of highly acid and oxidative compounds, which will destroy membranes made of common polymers. Since membranes made from perfluorinated thermoplastic polymers such as poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (POLY(PTFE-CO-PFVAE)) or poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) are not adversely affected by severe conditions of use, they have a decided advantage over membranes made from less chemically and thermally stable polymers.
Being chemically inert, the POLY(PTFE-CO-PFVAE) and FEP polymers are difficult to form into membranes using typical solution casting methods. They can be made into membranes using the Thermally Induced Phase Separation (TIPS) process. In one example of the TIPS process, a polymer and organic liquid are mixed and heated in an extruder to a temperature at which the polymer dissolves. A membrane is shaped by extrusion through an extrusion die, and the extruded membrane is cooled to form a gel. During cooling the polymer solution temperature is reduced to below the upper critical solution temperature. This is the temperature at or below which two phases form from the homogeneous heated solution, one phase primarily polymer, the other primarily solvent. If done properly, the solvent rich phase forms a continuous interconnecting porosity. The solvent rich phase is then extracted and the membrane dried. POLY(PTFE-CO-PFVAE) and FEP membranes made by the TIPS process are disclosed in U.S. Pat. No. 4,902,456, 4,906,377; 4,990,294; and 5,032,274. In the U.S. Pat. Nos. 4,902,456 and 4,906,377 patents, the membranes have a dense surface with either intervals of crack-like openings or pores, either singly, or as a series of several pores. The U.S. Pat. Nos. 4,990,294 and 5,032,274 patents disclose using a coating of the dissolution solvent on the shaped membrane as it exits the die. Both surfaces consist of a dense skin with porous areas. In one embodiment, membrane produced without co-extrusion in a sheet form is stretched in the transverse direction. The membrane surface for these membranes consists of nodular appearing structures separated by crack-like openings.
U.S. Pat. No. 5,395,570 discloses a method of extrusion of hollow fiber membranes in which a quadruple extrusion head is used to extrude a hollow fiber with a lumen-filling fluid, a coating layer, and a cooling fluid layer. This method requires a complex extrusion head and flow control means, and a separate coating layer consisting of the solvent between the cooling fluid and the extruded fiber. Also, the extruded fiber is not immediately contacted with the cooling fluid, but passes to a lower zone of the extrusion head before the fourth (cooling) layer is contacted with the coated fiber.
U.S. Pat. No. 4,564,488 discloses a process for preparing porous fibers and membranes. The process involves forming a homogeneous mixture of a polymer and at least another liquid inert with respect to the polymer. The mixture must have a temperature range of complete miscibility and a temperature range where there is a miscibility gap. The mixture is extruded at a temperature above the separation temperature into a bath preferably containing entirely or for the most part the inert liquid. The bath is maintained below the separation temperature. Disclosed but not claimed is an embodiment wherein the homogeneous mixture is extruded immediately into the bath containing entirely or for the most part the inert liquid, i.e., solvent No perfluorinated thermoplastic polymers are listed as xe2x80x9ccustomary polymersxe2x80x9d that are in the scope of the patent. No mention is made of special methods needed to extrude immediately into the cooling bath at very high temperatures.
WO 95/02447 discloses asymmetric PTFE membranes made by coating a solution of PTFE in a perfluorinated cycloalkane heated to about 340xc2x0 C. onto a substrate, removing the solvent and cooling the PTFE on the substrate so that one surface of the membrane is less porous than the other, and optionally, removing the substrate. No mention is made of applying this method to unsupported hollow fiber membranes.
U.S. Pat. No. 4,443,116 discloses a process for making a porous fluorinated polymer structure. Applicable polymers are copolymers of tetrafluoroethylene and perfluorovinylether with a sulfonyl fluoride (xe2x80x94SO2F), sulfonate (SO3Z) or carboxylate (COOZ) functional group wherein Z is a cation. The presence of the polar functional group greatly facilitates solubility. A thermally induced phase separation method is used in which the solvent must crystallize after cooling and phase separation. The solvent is removed while in a solid state. No pore structure or permeability data are given.
PTFE, POLY(PTFE-CO-PFVAE) and FEP sheet membranes are disclosed in U.S. Pat. No. 5,158,680, wherein an aqueous dispersion of PTFE with particles 1 micron or less and a filament forming polymer are mixed, formed into a membrane shape and heated to above the melting temperature and then the filament forming polymer is removed.
For filtration of ultrapure solutions, vanishingly low levels of extractable residual matter is required of the membrane. The TIPS process requires only the removal of the low molecular weight extrusion solvent after extrusion. This material is easily removed by extraction with a solvent, and since the POLY(PTFE-CO-PFVAE) and FEP material is inert to the extraction solvent, no change of membrane properties occurs. Extraction is simple and thorough due to the high porosity of the membrane and the high diffusion of the low molecular weight solvent. Membranes made by extraction of a polymer or resin would meet these requirements with extreme difficulty due to the inherent difficulty of removing the slowly diffusing polymers or resins.
Previous POLY(PTFE-CO-PFVAE) and FEP membranes made from the TIPS method required extrusion through an air gap. POLY(PTFE-CO-PFVAE) and FEP membranes made by the TIPS process are disclosed in U.S. Pat. Nos. 4,902,45; 4,906,377: 4,990,294; and 5,032,274. In the U.S. Pat. Nos. 4,902,456 and 4,906,377 patents, the membranes have a dense surface with either intervals of crack-like openings or pores, either singly, or as a series of several pores. The U.S. Pat. Nos. 4,990,294, and 5,032,274 patents disclose using a coating of the dissolution solvent on the shaped membrane as it exits the die. In one embodiment, the membrane in a sheet form is stretched in the transverse direction. It was found that the rapid evaporation of the solvent at the high extrusion temperatures gave skinning and poor control of the surface porosity. To overcome the skinning problems, a solvent coating method and post-stretching were employed by previous inventors. In the solvent coating method, the solvent, hot Halocarbon oil, heated to around 300xc2x0 C., is used to coat the melt surfaces as soon as the melt emerges from the die. While this method does suppress evaporation, it introduces other processing problems. First, it is very difficult to coat a melt surface uniformly with hot solvent because hot Halocarbon oil has the tendency to form droplets. Instead of a uniform coating, the solvent coating tends to streak along the melt surface. After the solution is cooled and solidified, the membrane surface shows uneven porosity due to non-uniform coating of solvent. Second, the temperature of the oil may not be uniform, and the resulting membrane would show high degree of variation of membrane properties due to uneven quenching of the surfaces. Third, the hot oil tends to soften the extruded melt and the extruded fiber tends to break apart during processing.
Post-stretching was disclosed as another technique to enhance permeability of a skinned PFA membrane in U.S. Pat. Nos. 4,990,294, and 5,032,274. While stretching does increase permeability substantially, it produces its own set of undesirable side effects. First, for stretching to be effective, the base skinned membrane must be very uniform in thickness and in mechanical strength. Any non-uniformity in the base membrane will be amplified as soon as the membrane is subjected to stretching, because weak areas stretch more than strong areas under the same stretching force. As mentioned above, it is very difficult to produce base membranes with the solvent coating technique. If solvent coating is not used, the heavy evaporation of porogen usually produces dried polymer on the die lips. This accumulated dried polymer then scratches the melt surfaces, producing lines of hidden weaknesses in the base membrane. Upon stretching, the weakened membranes break apart along the xe2x80x9cscratchxe2x80x9d lines.
It would therefore be desirable to have a process that would eliminate the rapid evaporation of solvent from the fiber surface, but not require a difficult coating or stretching step. It would also be beneficial to produce a skinless membrane having high surface porosity in order to utilize a large proportion of the membrane surface for permeation and retention.
It would further be desirable to have a porous hollow fiber contactor membrane for applications in which a highly soluble gas is to be transferred to a liquid having low interfacial tension.
This invention provides for high flux, skin-free hollow fiber porous membranes, more specifically, microporous membranes, from perfluorinated thermoplastic polymers, more specifically poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (POLY(PTFE-CO-PFVAE)) or poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP). These membranes are capable of operating in severe chemical environments with no apparent extractable matter being released. Compared to prior art membranes, the membranes of the invention have a higher surface porosity, which translates into high permeability or flux.
An embodiment of this invention provides for porous hollow fiber contactor membranes from perfluorinated thermoplastic polymers, more specifically poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (POLY(PTFE-CO-PFVAE)) or poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), and their use.
A process to produce these membranes is provided. The process is based on the Thermally Induced Phase Separation (TIPS) method of making porous structures and membranes. A mixture of polymer pellets, usually ground to a size smaller than supplied by the manufacturer, and a solvent, such as chlorotrifluoroethylene oligomer, is first mixed to a paste or paste-like consistency. The polymer comprises between approximately 12% to 35% by weight of the mixture. The solvent is chosen so the membrane formation occurs by liquid-liquid, rather than solid-liquid phase separation when the solution is extruded and cooled. Preferred solvents are saturated low molecular weight polymers of chlorotrifluoroethylene. A preferred solvent is HaloVac(copyright) 60 from Halocarbon Products Corporation, River edge, N.J. Choice of the solvent is dictated by the ability of the solvent to dissolve the polymer when heated to form an upper critical solution temperature solution, but not to excessively boil at that temperature. Fiber extrusion is referred to as spinning and the extruded fiber length from the die exit to the take-up station is referred to as the spin line. The paste is metered into a heated extruder barrel where the temperature raised to above the upper critical solution temperature so that dissolution occurs. The homogeneous solution is then extruded through an annular die directly into a liquid cooling bath with no air gap. The liquid cooling bath is maintained at a temperature below the upper critical solution temperature of the polymer solution. The preferred bath liquid is not a solvent for the thermoplastic polymer, even at the extrusion temperature. Upon cooling, the heated and shaped solution undergoes phase separation and a gel fiber results. The die tip is slightly submerged for vertical spinning, i.e.; the spin line falls downward, in the direction of a freely failing body. For horizontal spinning, where the spin line exits directly in the horizontal attitude, and is maintained more or less in that plane until at least the first guide roll, a specially design die is used. The die is firmly positioned against an insulated wall with the die tip penetrating through an opening having a liquid-tight seal in the insulator wall. A trough for cooling liquid flow is placed in a recess in the opposite side of the insulating wall, in a manner that will maintain the die nose outlet in a submerged condition. Cooling liquid flows in the trough and overflows in a region of the trough of lesser depth, keeping the die nose outlet submerged with a flow of cooling liquid. In both the vertical and horizontal methods, a booster heater and temperature control means is used to briefly raise the solution temperature at the die tip to prevent premature cooling. In a subsequent step, the dissolution solvent is removed by extraction and the resultant hollow fiber membrane is dried under restraint to prevent membrane shrinkage and collapse. Optionally the dried fiber may be heat set at 200xc2x0 C. to 300xc2x0 C.